One of the biggest challenges when building your own environmental monitoring instrumentation is how to get the sensor cable into the waterproof box with the logger. There are several different methods, but I prefer plastic or nylon cable glands. You can get them from a variety of vendors and for many different diameters of cables. They also make ones that allow you to run anywhere from 2 to 6 or more wires into one gland, but those are more complicated to use. In most cases, just the single-cable ones will work. You can get them from most industrial suppliers and electronics companies, but even Amazon has some like these for medium cables, and these for small cables.
One thing to keep in mind is the amount of threading on the gland. If your enclosure isn’t too thick, most cable glands will work. But if you’re mounting them on a box that has thick walls, you might need to buy glands with longer threads. To drill your enclosure, I find that step drill bits work best. Just be sure not to remove too much material, otherwise the gland won’t seal properly against the enclosure and you’ll get leaks.